The use of EMAT assemblies for inspecting welds is known from U.S. Pat. Nos. 5,439,157 and 5,474,225. In the known EMAT weld inspection methods a robotic transport apparatus containing EMAT transmitting and receiving coils is automatically positioned at one side of a just-completed weld whereupon the EMAT transmitting coil transmits ultrasonic SH shear waves towards the weld and the EMAT receiving coil transduces any ultrasonic SH shear waves reflected by the weld in a signal which is used to signal the presence of defects in the weld on the basis of the received signal. The robotic transport apparatus is in use moved along the surface of one of the welded plates parallel to the weld and may be connected to a control unit which automatically adjusts the settings of the welding apparatus which moves ahead of the EMAT weld assembly. The use of a robotic transport apparatus is not practical for inspection of welds between tubulars since it requires the robotic transport apparatus to rotate around a welded tubular, which is time consuming and requires the use of a fragile robotic tool.
The use of EMAT devices for weld and/or pipe inspection is also disclosed in U.S. Pat. No. 5,652,389 to Barnes, et. al., U.S. Pat. No. 5,760,307 to Latimer et. al., WO Patent No. 02/40986 and U.S. Pat. No. 5,808,202 to Passarelli. Barnes discloses a pulse-echo technique and apparatus for inspection of inertia welds in plat-plates using EMAT. Latimer discloses a method to eliminate root and crown signals using crossed or collinear EMATs, and Passarelli discloses a pulse-echo technique for the inspection of cylindrical objects including rods and tubes.
The device disclosed by Passarelli has the disadvantage that it is has a fixed ring-shape construction, which cannot be put readily around the tubulars and the weld at the rig floor without the danger of damaging the device or at the expense of substantial time delays. Another disadvantage to this arrangement is the geometry of the electromagnets, the transmitter and the receiver coil, which does not provide a 100% inspection of the weld around the circumference of the pipe, as the aperture of the transmitters is smaller than the ultrasonic field at the weld region. Rotating the tubular could mitigate the disadvantage, but that is not possible when the tubulars are welded at the rig floor, as will be explained below. An additional, difficulty posed by this and other prior art is that the weld is inspected by pulse-echo reflection measurement only. However, to prevent miss-interpretation of the reflected signals, e.g. due to diffraction or scatter at the weld, it is preferred to measure both reflection and transmission at the same time using at least two EMATs positioned upstream and downstream from the weld.